1. Field of the Invention
This invention relates generally to thermometry and more specifically, to an infrared radiation thermometer.
2. Description of the Prior Art
The temperature of an object, such as human body, can be determined by two methods: either by a contact thermometer having a probe which comes into physical contact with the measured object, or by a non-contact thermometer which measures infrared (IR) radiation exchange between its radiation detector and an object (target). This invention relates to the latter method.
Variations in internal temperature of an infrared thermometer affect its accuracy and must always be taken into account when the temperature of an object is measured.
One method to reduce variation in measurement is to maintain the thermometer's interior temperature constant by use of a thermostat as described in U.S. Pat. No. 4,602,642 issued to O'Hara et al.
Another method is to control the temperature of the sensor's surface, maintaining it at a predetermined level as in U.S. Pat. No. 4,854,730 issued to Fraden.
Still another method is to control temperature of the sensor's housing at a level which brings the thermal detector output signal essentially to zero level as in U.S. Pat. No. 4,900,162 issued to Beckman et al. The above mentioned U.S. Pat. No. 4,854,730 issued to Fraden and U.S. Pat. No. 4,634,294 issued to Christo et al. teach compensation for ambient temperature variations by using an additional ambient temperature sensor, the signal of which is used for calculation of the correction.
There is another method of compensating for ambient temperature variations by using a symmetrical sensor design, where one thermal detector is exposed to infrared radiation and the other thermal detector is shielded from it. Such an approach is described in U.S. Pat. No. 3,023,398 issued to Siegert. An obvious advantage of this approach is mutual compensation for many instabilities, such as thermal drifts, aging, etc. On the other hand, a symmetrical sensor cannot compensate for instabilities in its components which are related only to the IR radiation, such as optical filters, windows or mirrors. Another problem is that the generally different operating temperatures of both detectors makes measurement less accurate.
In another method for compensating for ambient temperature variation, the same radiation detector for the object target, measures infrared radiation from a reference target of known temperature. Such an approach is covered, for example by the above cited U.S. Pat. Nos. 4,797,840, 4,602,642 and by U.S. Pat. No. 4,005,605 issued to Michael.
Other problems in measuring IR radiation include non-linearity, thermal drifts and aging of many infrared detectors. Such problems may be overcome by use of a null radiometer like that which is mentioned in U.S. Pat. No. 4,900,162 issued to Beckman et al. In that patent, temperature of a thermal radiation detector housing is actively controlled so that the base line signal from the detector is essentially equal to zero. Hence, the IR detector's own characteristics have less impact on overall accuracy.
The present invention is designed to provide an infrared thermometer which is less sensitive to variations in ambient temperature, and less susceptible to variabilities inherent in internal elements.
It is one object of the present invention to provide a symmetrical infrared sensor.
It is another object of the invention to provide an infrared thermometer with symmetrical sensor, and a thermal reference source.
Another object is to provide an infrared thermometer with symmetrical sensor, thermal reference source, and a null type radiometer.
Additional objects and advantages will become obvious from the ensuing descriptions of the invention.
Briefly, the present invention provides a radiation thermometer with a sensor that includes two thermal energy detectors, thermally insulated from one another.
The first detector is warmed up or cooled down by infrared radiation between the object and the first detector, while the second detector is shielded from radiation from the object, and is warmed up or cooled down by an internal reference temperature source.
Each detector provides a signal that is representative of the temperature of the detector.
The two detectors are brought substantially to a known temperature ratio, preferably to the same temperature, by heater/cooler means. A heater/cooler means changes the temperature of an item by exchanging thermal energy with the item.
The temperature of the target can be determined when the detectors of the sensor are in a balanced state which means, that heat received from the reference source is a known ratio to that received from the object target.
When the first detector is subjected to infrared radiation from a target, a control circuit which receives signals from the detectors operates the heating or cooling reference temperature source that brings the temperature of the second detector to the known temperature ratio with the first detector by exchange of thermal or heat energy between the heater/cooler source and the second detector. When the detectors are in balance, that is when the detector temperature ratio is established, the temperature, preferably of the reference source, is measured by a third temperature detector, and a processor which receives a signal from the third detector calculates the temperature of the object target.